Expandable styrene polymers, and aromatic-resistant foams produced therefrom

ABSTRACT

Expandable styrene polymers which contain from 10 to 90% by weight of a crosslinked styrene-diene copolymer containing from 2 to 45% by weight of the diene, and from 1 to 10% by weight of a C3-to C6-hydrocarbon as blowing agent can be processed to give foams which have high resistance to aromatics.

This is a division of application Ser. No. 07/606,314, filed on Oct. 31,1990.

The present invention relates to novel expandable styrene polymers forthe production of aromatic-resistant foams.

Foams based on styrene polymers have achieved considerable industrialimportance as heat-insulating and packaging materials. Due to their lackof resistance to aromatics, however, use of these foams in automobileconstruction is limited.

It is an object of the present invention to develop aromatic-resistantpolystyrene foams.

We have found that, surprisingly, this object is achieved by mixture ofpolystyrene and crosslinked styrene-diene copolymers which givearomatic-resistant foams although the principal constituent polystyreneis soluble in aromatics.

The invention accordingly provides expandable styrene polymers whichcontain

a) from 10 to 90% by weight of uncrosslinked polystyrene and/or anuncrosslinked copolymer containing 50% by weight or more ofcopolymerized styrene,

b) from 10 to 90% by weight of a crosslinked styrene-diene copolymercontaining from 2 to 45% by weight of diene,

c) from 1 to 10% by weight, based on the sum of a) and b), of a C₃ - toC₆ -hydrocarbon as blowing agent, and, if desired,

d) conventional additives in effective amounts.

The invention furthermore relates to a process for the preparation ofexpandable styrene polymers of this type which comprises dissolving anuncrosslinked styrene-diene copolymer containing from 10 to 90% byweight of diene in styrene, polymerizing the material in aqueoussuspension, and adding the blowing agent and, if used, the additivesbefore, during or after the polymerization.

The invention furthermore provides foams having a density of from 0.005to 0.1 g/cm³ and containing

a) from 10 to 90% by weight of polystyrene and/or a copolymer containing50% by weight or more of copolymerized styrene,

b) from 10 to 90% by weight of a crosslinked styrene-diene copolymercontaining from 2 to 45% by weight of diene, and, if desired,

c) conventional additives in effective amounts.

The principal component a) in the novel products comprises from 10 to90% by weight, preferably from 20 to 80% by weight, in particular from30 to 70% by weight, of polystyrene and/or a styrene copolymercontaining 50% by weight or more, preferably 80% by weight or more, ofcopolymerized polystyrene. Examples of suitable comonomers areα-methylstyrene, ring-halogenated styrenes, ring-alkylated styrenes,acrylonitrile, esters of acrylic or methacrylic acid with alcoholshaving from 1 to 8 carbon atoms, N-vinylcarbazole, maleic acid andmaleic anhydride. The polystyrene advantageously contains, inpolymerized form, a small amount of a crosslinking agent which resultsin chain branching, i.e. a compound containing more than one, preferably2, double bonds, such as divinylbenzene, butadiene or butanedioldiacrylate. The crosslinking agent is generally used in amounts of from0.005 to 0.05 mol. %, based on styrene. Component a) is uncrosslinked,i.e. is soluble in boiling toluene.

In order to achieve particularly high expandability, it is expedient forthe styrene polymer to have a mean molecular weight M_(w) (weightaverage), measured by the GPC method, of from 100,000 to 200,000, inparticular from 130,000 to 180,000. The foam has improved processingproperties if the high-molecular-weight edge of the molecular-weightdistribution curve, measured by the GPC method, is so steep that thedifference between the means (M_(z+1) -M_(z)) is less than 150,000. TheGPC method is described in G. Glockler, Polymer-charakterisierung,Chromatographische Methoden Volume 17, Huthig-Verlag, Heidelberg, 1982.The means mentioned are described in H.G. Elias, Makromolekule,Huthig-Verlag, Heidelberg, 1971, pages 52-64.

The styrene polymers which have the above-mentioned mean molecularweights can be obtained by carrying out the polymerization in thepresence of regulators, expediently from 0.01 to 1.5% by weight,preferably from 0.01 to 0.5% by weight, of a bromine-free organiccompound having a chain-transfer constant K of from 0.1 to 50. In orderto achieve a steep high-molecular-weight edge of the molecular-weightdistribution curve, the regulator is expediently not added until aconversion of from 20 to 90% has been achieved in the polymerization.

An advantageous high expansion capacity can also be achieved ifcomponent a) contains from 0.1 to 10% by weight, advantageously from 0.5to 10% by weight, of a styrene polymer having a mean molecular weight(weight average) of from 500 to 5,000.

Further details on molecular weight regulation in the preparation ofexpandable styrene polymers are given in EP-B 106 129.

Styrene polymers which contain from 0.1 to 2% by weight, preferably from0.15 to 1.5% by weight, of copolymerized acrylonitrile give foams whichare substantially free from shrinkage. A mixture of from 95 to 99.5% byweight of polystyrene and from 0.5 to 5% by weight of a styrene-solublestyrene acrylonitrile copolymer also has these properties if the totalacrylonitrile content in the mixture is from 0.1 to 2% by weight,preferably from 0.15 to 2% by weight.

Styrene polymers which contain from 2 to 15% by weight, in particularfrom 3 to 12% by weight, of maleic acid or maleic anhydride as comonomergive foams with high heat distortion resistance. The starting materialhere is advantageously a mixture of polystyrene and a commerciallyavailable styrene-maleic anhydride copolymer containing from 15 to 49%by weight of maleic anhydride; this can easily be prepared by dissolvingthe copolymer in styrene followed by polymerization.

As component b) which is essential to the invention, the novel productscontain from 10 to 90% by weight, preferably from 20 to 80% by weight,in particular from 30 to 70% by weight, of a crosslinked styrene-dienecopolymer containing from 2 to 45% by weight, preferably from 3 to 30%by weight, in particular from 5 to 20% by weight, of the diene. Examplesof suitable products are styrene-isoprene copolymers, and in particularstyrene-butadiene copolymers. The copolymers either have a random oradvantageously a block structure. Graft copolymers are particularlysuitable. The copolymers are distributed uniformly in the polystyrenematrix, presumably essentially as a network. A distribution of this typeis obtained, for example, by starting from an uncrosslinked,styrene-soluble copolymer and polymerizing the styrene solution, duringwhich the copolymer crosslinks while styrene molecules aresimultaneously grafted onto the copolymer.

Component b) is crosslinked, i.e. insoluble in boiling toluene. Theproportion of crosslinked component b) in the mixture of a) and b) caneasily be measured by extracting the mixture with boiling toluene anddetermining the toluene-insoluble residue.

The mixture of components a) and b) should expediently have a melt flowindex MFI 200/5.0 (in accordance with DIN 53 735) of from 0.2 to 15,advantageously from 0.3 to 8, in particular from 1 to 5 [g/10 min]. Ifthe melt flow index is too low, the expandability is relatively low, andif the index is too high, the aromatics-resistance of the foam isunsatisfactory.

As blowing agents, the expandable styrene polymers contain from 1 to 10%by weight, preferably from 3 to 8% by weight, in particular from 5 to 8%by weight, based on the sum of a) and b), of a C₃ - to C₆ -hydrocarbon,such as propane, butane, isobutane, n-pentane, isopentane, neopentaneand/or hexane. Preference is given to a commercially available pentanemixture.

The expandable styrene polymers may furthermore contain conventionaladditives in effective amounts, such as dyes, fillers, stabilizers,flameproofing agents, synergists, nucleating agents, lubricants,antistatics, substances which have an anti-adhesive effect on foaming,and agents for shortening the demolding time after final foaming.

Other suitable additives are poly(2,6-dimethyl)-1,4-phenylene ether andpoly-1,4-phenylene sulfide. In amounts of from 1 to 20% by weight, basedon component a), these additives increase the heat distortion resistanceof the foam.

The expandable styrene polymers are generally in the form of particles,i.e. in the form of beads, granules or lumps and advantageously have amean diameter of from 0.1 to 6 mm, in particular from 0.4 to 3 mm.

They can be prepared, for example, by mixing the components in the meltin an extruder, cooling the extrudate so rapidly that expansion does notoccur, and subsequently comminuting the extrudate.

Preference is given to preparation by suspension polymerization in aconventional manner. To this end, the uncrosslinked styrene-dienecopolymer and, if used, the additives are dissolved in styrene, and thissolution is polymerized in aqueous suspension. A regulator and/or asmall amount, from about 0.005 to 0.05 mol. %, based on styrene, of acrosslinking agent are advantageously added during the polymerization.The blowing agent can be added to the batch before, during or after thepolymerization.

The uncrosslinked, styrene-soluble styrene-diene copolymer used toprepare the component b) distributed in a) is prepared in a conventionalmanner by copolymerizing styrene and a diene, such as butadiene orisoprene. It should contain from 10 to 90% by weight, advantageouslyfrom 15 to 80% by weight, in particular from 20 to 60% by weight, ofcopolymerized diene. Preference is given to styrene-butadiene blockcopolymers prepared by anionic polymerization. Branched or stellateblock copolymers which are produced by coupling living polymersobtainable using organoalkali metal catalysts with multifunctionalcoupling agents and which contain, for example, from 20 to 45% by weightof the diene and have a viscosity number VN of from 50 to 130 [ml/g](measured in 0.5% strength toluene solution) are preferred. Processesfor the preparation of block copolymers of this type are described inU.S. Pat. No. 3,281,383 and British Patent 985,614. Crosslinking of thecopolymer takes place during the polymerization of the styrene solutionof the styrene-diene copolymer, and further styrene is generally graftedon at the same time. The proportion of cross-linked styrene-dienecopolymer b) in the mixture of a) and b) is therefore generally greaterthan the amount of styrene-soluble uncrosslinked copolymer employed andits content of copolymerized diene is generally lower.

When the polymerization is complete, the bead-form expandable styrenepolymers obtained are separated off from the aqueous phase, washed anddried.

To produce foams, the expandable styrene polymers are expanded in aconventional manner by heating to above their softening point, forexample using hot air or preferably using steam. After cooling andpossibly after interim storage, the foam particles obtained can bere-expanded by reheating. They can subsequently be welded in aconventional manner in molds which do not close gas-tight to givemoldings.

The foams obtained have a density of from about 0.005 to 0.1 g/cm³,preferably from 0.01 to 0.05 g/cm³. Due to their elasticity, they areused, in particular, for shock-absorbent packaging, as the core materialfor automotive fenders, for internal paneling of motor vehicles, as acushioning material, and as a heat- and sound-insulating material.

In the examples, parts are by weight.

EXAMPLES 1 TO 8

A mixture of 200 parts of water, 0.1 part of sodium pyrophosphate, 100parts of a styrene solution containing the amounts indicated in thetable of a branched styrene-butadiene star block copolymer (75/25% byweight), viscosity number 75 (ml/g), and the amounts indicated in thetable of divinylbenzene, 7.0 parts of pentane, 0.15 part of tert-butylperoxide, 0.45 part of benzoyl peroxide and 4 parts of a 10% strengthaqueous solution of polyvinylpyrrolidone is heated to 90° C. withstirring in a pressure-tight stirred reactor, kept at 90° C. for 5 hoursand subsequently heated at 100° C. for 2 hours and at 120° C. for afurther 2 hours. After cooling, the bead polymer obtained having a meanparticle diameter of about 1 mm was separated off from the aqueousphase, washed and dried.

The expandable styrene polymer obtained was prefoamed in a commerciallyavailable Rauscher-type stirred prefoamer, using flowing steam to a bulkdensity of about 20 g/l. After interim storage for 24 hours, the foamparticles were welded in a Rauscher-type block mold by steam treatmentat 1.8 bar to give a block of density about 20 g/l.

To test the oil- and aromatic-resistance, 5 cubes of the foam for eachtest, with an edge length of 5 cm, were stored for 72 hours in heatingoil and toluene in accordance with DIN 53 428. The evaluation criteriaare 0=unchanged, 1=slightly changed, 2=non-resistant.

In order to determine the content of crosslinked styrene-dienecopolymer, a degassed sample which had been freed from blowing agent waspowdered and extracted for 24 hours with boiling toluene in a Soxhletextractor, and the toluene-insoluble content was determined.

The results are given in the table.

                                      TABLE                                       __________________________________________________________________________    Starting material [% by weight]      Melt flow index                                    Styrene-   Component a)                                                                          Component b)                                                                          MFI 200/5.0                                                                           Resistance                                 butadiene  (toluene-                                                                             (toluene-                                                                             DIN 53 735                                                                            of the foam                                star block                                                                          Divinyl-                                                                           soluble)                                                                              insoluble)                                                                            of the mixture                                                                        in    in                         Example                                                                            Styrene                                                                            copolymer                                                                           benzene                                                                            [% by weight]                                                                         [% by weight]                                                                         of a) and b)                                                                          heating oil                                                                         toluene                    __________________________________________________________________________    1    83   17    --   50      50      4.0     0     1                          2    83   17    0.01 35      65      2.1     0     0                          3    77   23    --   24      76      1.9     0     0                          4    77   23    0.01 20      80      1.5     0     0                          5    74   26    --   17      83      1.7     0     0                          6    74   26    0.01 10      90      0.4     0     0                          7 (comp.)                                                                          100  --    --   100     --      9.9     2     2                          8 (comp.)                                                                          100  --    0.01 100     --      9.2     2     2                          __________________________________________________________________________

We claim:
 1. A foam which has a density of from 0.005 to 0.1 g/cm³ andcontainsa) from 10 to 90% by weight of uncrosslinked polystyrene and/oran uncrosslinked styrene copolymer containing 50% by weight or more ofcopolymerized styrene, b) from 10 to 90% by weight of a crosslinkedstyrene-diene copolymer containing from 2-45% of diene, and, optionally,c) conventional additives in effective amounts.
 2. A process for theproduction of a foam as claimed in claim 1, which comprises expanding anexpandable styrene polymer which containsa) from 10 to 90% by weight ofuncrosslinked polystyrene and/or an uncrosslinked styrene copolymercontaining 50% by weight or more of copolymerized styrene, b) from 10 to90% by weight of crosslinked styrene-diene copolymer containing from 2to 45% by weight of the diene, c) from 1 to 10% by weight, based on thesum of a) and b), of a C₃ - to C₆ -hydrocarbon as blowing agent, andoptionally, d) conventional additives in effective amounts, wherein saidexpandable styrene polymer is in particulate form by heating to abovethe softening point, and welding the foam particles obtained to oneanother by heating in molds which do not close gas-tight.